1. Field of the Invention
The present invention relates to a structure of a coaxial-to-waveguide transition for an input and/or output of radio frequency signals, and a traveling wave tube including the structure of the coaxial-to-waveguide transition.
2. Description of the Related Art
Conventionally, a traveling wave tube is known as a microwave tube. Many traveling wave tubes include structures of coaxial-to-waveguides transition as input window in which a radio frequency wave is inputted, or output window from which a radio frequency wave is outputted.
The output window included in a traveling wave tube related to the present invention is disclosed, for example, in Japanese Utility Model Laid-Open No. 5-23397 (see FIG. 1). As shown in FIG. 1, output transition section 101 included by the traveling wave tube related to the present invention is configured by including waveguide 106 for outputting radio frequency wave, vacuum envelope 107 provided with slow-wave circuit 108 in the interior of the vacuum, insulating window member 109 which hermetically seals a side of vacuum envelope 107 and a side of waveguide 106, a coaxial connection portion 110 which connects the waveguide 106 and the vacuum envelope 107, coaxial center conductor of exterior portion 111 with one end supported by waveguide 106, and coaxial center conductor of interior portion 112 with one end abutting on slow-wave circuit 108 and the other end connected to the coaxial center conductor of exterior portion 111.
In such a traveling wave tube, the matching property in the vicinity of insulating window member 109 is determined by the characteristic impedance set by the size of the component parts including the coaxial center conductor of exterior portion 111. In the output transition section, in order to reduce the return loss of an amplified radio frequency wave, impedance in the output transition section needs to be adjusted to be optimal.
Incidentally, each of the components configuring the output transition section inevitably causes variation in the outside dimensions, such as the length and the outside diameter due to machining accuracy, dimensional tolerance and the like. Therefore, in the configuration of the output transition section related to the present invention, in order to adjust the characteristic impedance to a desired optimal value, a plurality of components differing in outside dimensions are prepared when manufacturing the individual output transition sections, and the components from which the optimal impedance value is obtained are selected and assembled from a plurality of components. Therefore, there are disadvantages in that the operation of adjusting impedance is complicated, and manufacturing costs increase.